Homeostasis

Homeostasis

Overview
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH. It can be either an open or closed
Closed system
-In physics:In thermodynamics, a closed system can exchange energy , but not matter, with its surroundings.In contrast, an isolated system cannot exchange any of heat, work, or matter with the surroundings, while an open system can exchange all of heat, work and matter.For a simple system, with...

 system.

It was defined by Claude Bernard
Claude Bernard
Claude Bernard was a French physiologist. He was the first to define the term milieu intérieur . Historian of science I. Bernard Cohen of Harvard University called Bernard "one of the greatest of all men of science"...

 and later by Walter Bradford Cannon
Walter Bradford Cannon
Walter Bradford Cannon, M.D. was an American physiologist, professor and chairman of the Department of Physiology at Harvard Medical School. He coined the term fight or flight response, and he expanded on Claude Bernard's concept of homeostasis...

 in 1926, 1929 and 1932.

Typically used to refer to a living organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

, the concept came from that of milieu interieur
Milieu interieur
Milieu intérieur or interior milieu, from the French, milieu intérieur, is a term coined by Claude Bernard to refer to the extra-cellular fluid environment, and its physiological capacity to ensure protective stability for the tissues and organs of multicellular living organisms.-Origin:Claude...

 that was created by Claude Bernard
Claude Bernard
Claude Bernard was a French physiologist. He was the first to define the term milieu intérieur . Historian of science I. Bernard Cohen of Harvard University called Bernard "one of the greatest of all men of science"...

 and published in 1865.
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Encyclopedia
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH. It can be either an open or closed
Closed system
-In physics:In thermodynamics, a closed system can exchange energy , but not matter, with its surroundings.In contrast, an isolated system cannot exchange any of heat, work, or matter with the surroundings, while an open system can exchange all of heat, work and matter.For a simple system, with...

 system.

It was defined by Claude Bernard
Claude Bernard
Claude Bernard was a French physiologist. He was the first to define the term milieu intérieur . Historian of science I. Bernard Cohen of Harvard University called Bernard "one of the greatest of all men of science"...

 and later by Walter Bradford Cannon
Walter Bradford Cannon
Walter Bradford Cannon, M.D. was an American physiologist, professor and chairman of the Department of Physiology at Harvard Medical School. He coined the term fight or flight response, and he expanded on Claude Bernard's concept of homeostasis...

 in 1926, 1929 and 1932.

Typically used to refer to a living organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

, the concept came from that of milieu interieur
Milieu interieur
Milieu intérieur or interior milieu, from the French, milieu intérieur, is a term coined by Claude Bernard to refer to the extra-cellular fluid environment, and its physiological capacity to ensure protective stability for the tissues and organs of multicellular living organisms.-Origin:Claude...

 that was created by Claude Bernard
Claude Bernard
Claude Bernard was a French physiologist. He was the first to define the term milieu intérieur . Historian of science I. Bernard Cohen of Harvard University called Bernard "one of the greatest of all men of science"...

 and published in 1865. Multiple dynamic equilibrium adjustment and regulation mechanisms make homeostasis possible.

Biological


With regards to any given life system parameter
Parameter
Parameter from Ancient Greek παρά also “para” meaning “beside, subsidiary” and μέτρον also “metron” meaning “measure”, can be interpreted in mathematics, logic, linguistics, environmental science and other disciplines....

, an organism may be a conformer or a regulator. On one hand, regulators try to maintain the parameter at a constant level over possibly wide ambient environmental variations. On the other hand, conformers allow the environment to determine the parameter. For instance, endotherm
Endotherm
An endotherm is an organism that produces heat through internal means, such as muscle shivering or increasing its metabolism...

ic animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s (mammals and birds) maintain a constant body temperature, while ectotherm
Ectotherm
An ectotherm, from the Greek εκτός "outside" and θερμός "hot", refers to organisms that control body temperature through external means. As a result, organisms are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body...

ic animals (almost all other organisms) exhibit wide body temperature variation.

Behavioral adaptations allow ectothermic animals to exert some control over a given parameter. For instance, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s often rest on sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

-heated rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s in the morning to raise their body temperature. Regulators are also responsive to external circumstances, however: if the same sun-baked boulder happens to host a ground squirrel, the animal's metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 will adjust to the lesser need for internal heat production.

An advantage of homeostatic regulation is that it allows an organism to function effectively in a broad range of environmental conditions. For example, ectotherms tend to become sluggish at low temperatures, whereas a co-located endotherm may be fully active. That thermal stability comes at a price since an automatic regulation system requires additional energy. One reason snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...

s may eat only once a week is that they use much less energy to maintain homeostasis.

Most homeostatic regulation is controlled by the release of hormones into the bloodstream. However, other regulatory processes rely on simple diffusion to maintain a balance.

Homeostatic regulation extends far beyond the control of temperature. Homeostasis includes regulation of the pH of the Blood at 7.365 (a measure of alkalinity and acidity). All animals also regulate their blood glucose, as well as the concentration of their blood. Mammals regulate their blood glucose with insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....

 and glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...

. The human body maintains glucose levels constant most of the day, even after a 24-hour fast. Even during long periods of fasting, glucose levels are reduced only very slightly. Insulin, secreted by the beta cells of the pancreas, effectively transports glucose to the body's cells by instructing those cells to keep more of the glucose for their own use. See Dynamic equilibrium
Dynamic equilibrium
A dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state...

. If the glucose inside the cells is high, the cells will convert it to the insoluble glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...

 to prevent the soluble glucose from interfering with cellular metabolism. Ultimately this lowers blood glucose levels, and Insulin helps to prevent hyperglycemia
Hyperglycemia
Hyperglycemia or Hyperglycæmia, or high blood sugar, is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a glucose level higher than 13.5mmol/l , but symptoms may not start to become noticeable until even higher values such as 15-20 mmol/l...

. When insulin is deficient or cells become resistant to it, diabetes
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...

 occurs. Glucagon, secreted by the alpha cells of the pancreas, encourages cells to break down stored glycogen or convert non-carbohydrate carbon sources to glucose via gluconeogenesis
Gluconeogenesis
Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids....

, thus preventing hypoglycemia
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...

. The kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

s are used to remove excess water and ions from the blood. These are then expelled as urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

. The kidneys perform a vital role in homeostatic regulation in mammals, removing excess water, salt, and urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

 from the blood. These are the body's main waste products.

Another homeostatic regulation occurs in the gut. Homeostasis of the gut is not fully understood but it is believed that Toll-like receptor
Toll-like receptor
Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single, membrane-spanning, non-catalytic receptors that recognize structurally conserved molecules derived from microbes...

 (TLR) expression profiles contribute to it. Intestinal epithelial cells exhibit important factors that contribute to homeostasis: 1) They have different cellular distribution of TLR's compared to the normal gut mucosa. An example of this is how TLR5
TLR 5
Toll-like receptor 5, also known as TLR5, is a protein which in humans is encoded by the TLR5 gene.- Function :The protein encoded by this gene is a member of the Toll-like receptor family which plays a fundamental role in pathogen recognition and activation of innate immunity...

 (activated by flagellin) can redistribute to the basolateral membrane, which is the perfect place where flagellin can be detected. 2) The enterocytes express high levels of TLR inhibitor Toll-interacting protein (TOLLIP
TOLLIP
Toll interacting protein, also known as TOLLIP, is an inhibitory adaptor protein that in humans is encoded by the TOLLIP gene.-Function:It is an inhibitory adaptor protein within Toll-like receptors...

). TOLLIP is a human gene that is a part of the innate immune system
Innate immune system
The innate immune system, also known as non-specific immune system and secondary line of defence, comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner...

 and is highest in a healthy gut; it correlates to luminal bacterial load. 3) Surface enterocytes also express high levels of Interleukin-1 receptor
Interleukin-1 receptor
Interleukin-1 receptor is a cytokine receptor which binds interleukin 1. Two forms of the receptor exist. The type I receptor is primarily responsible for transmitting the inflammatory effects of interleukin-1 while type II receptors may act as a suppressor of IL-1 activity by competing for...

 (IL-1R) -containing inhibitory molecule. IL-1R are also referred to as single immunoglobulin IL-1R (SIGIRR). Animals deficient in this are more susceptible to induced colitis, implying that SIGIRR might possibly play a role in tuning mucosal tolerance towards commensal flora. Nucleotide-binding oligomer
Oligomer
In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...

isation domain containing 2 (NOD2) is suggested to have an effect on suppressing inflammatory cascades based on recent evidence. It is believed to modulate signals transmitted through TLRs, TLR3, 4, and 9 specifically. Mutation of it has resulted in Crohn's disease
Crohn's disease
Crohn's disease, also known as regional enteritis, is a type of inflammatory bowel disease that may affect any part of the gastrointestinal tract from mouth to anus, causing a wide variety of symptoms...

. Excessive T-helper 1 responses to resident flora in the gut are controlled by inhibiting the controlling influence of regulatory T cells and tolerance-inducing dendritic cells.

Sleep timing depends upon a balance between homeostatic sleep propensity, the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode, and circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s that determine the ideal timing of a correctly structured and restorative sleep episode.

Control mechanisms


All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: The receptor is the sensing component that monitors and responds to changes in the environment. When the receptor senses a stimulus, it sends information to a "control center", the component that sets the range at which a variable is maintained. The control center determines an appropriate response to the stimulus. In most homeostatic mechanisms, the control center is the brain. The control center then sends signals to an effector, which can be muscles, organs or other structures that receive signals from the control center. After receiving the signal, a change occurs to correct the deviation by either enhancing it with positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

 or depressing it with negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

.

Positive feedback



Positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

 mechanisms are designed to accelerate or enhance the output created by a stimulus that has already been activated.

Unlike negative feedback mechanisms that initiate to maintain or regulate physiological functions within a set and narrow range, the positive feedback mechanisms are designed to push levels out of normal ranges. To achieve this purpose, a series of events initiates a cascading process that builds to increase the effect of the stimulus. This process can be beneficial but is rarely used by the body due to risks of the acceleration's becoming uncontrollable.

One positive feedback example event in the body is blood platelet
Platelet
Platelets, or thrombocytes , are small,irregularly shaped clear cell fragments , 2–3 µm in diameter, which are derived from fragmentation of precursor megakaryocytes.  The average lifespan of a platelet is normally just 5 to 9 days...

 accumulation, which, in turn, causes blood clotting in response to a break or tear in the lining of blood vessels. Another example is the release of oxytocin
Oxytocin
Oxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain.Oxytocin is best known for its roles in sexual reproduction, in particular during and after childbirth...

 to intensify the contractions that take place during childbirth.

Negative feedback


Negative feedback mechanisms consist of reducing the output or activity of any organ or system back to its normal range of functioning. A good example of this is regulating blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

. Blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...

s can sense resistance of blood flow against the walls when blood pressure increases. The blood vessels act as the receptors and they relay this message to the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. The brain then sends a message to the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 and blood vessels, both of which are the effectors. The heart rate would decrease as the blood vessels increase in diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 (known as vasodilation
Vasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...

). This change would cause the blood pressure to fall back to its normal range. The opposite would happen when blood pressure decreases, and would cause vasoconstriction
Vasoconstriction
Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, particularly the large arteries, small arterioles and veins. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in...

.

Another important example is seen when the body is deprived of food. The body would then reset the metabolic set point to a lower than normal value. This would allow the body to continue to function, at a slower rate, even though the body is starving. Therefore, people who deprive themselves of food while trying to lose weight would find it easy to shed weight initially and much harder to lose more after. This is due to the body readjusting itself to a lower metabolic set point to allow the body to survive with its low supply of energy. Exercise can change this effect by increasing the metabolic demand.

Another good example of negative feedback mechanism is temperature control. The hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

, which monitors the body temperature, is capable of determining even the slightest variation of normal body temperature (37 degrees Celsius
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...

). Response to such variation could be stimulation of glands that produce sweat to reduce the temperature or signaling various muscles to shiver to increase body temperature.

Both feedbacks are equally important for the healthy functioning of one's body. Complications can arise if any of the two feedbacks are affected or altered in any way.

Homeostatic imbalance


Many diseases are a result of disturbance of homeostasis, a condition known as homeostatic imbalance. As it ages, every organism will lose efficiency in its control systems. The inefficiencies gradually result in an unstable internal environment that increases the risk for illness. In addition, homeostatic imbalance is also responsible for the physical changes associated with aging. Even more serious than illness and other characteristics of aging is death. Heart failure has been seen where nominal negative feedback mechanisms become overwhelmed, and destructive positive feedback mechanisms then take over.

Diseases that result from a homeostatic imbalance include diabetes, dehydration
Dehydration
In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

, hypoglycemia, hyperglycemia, gout
Gout
Gout is a medical condition usually characterized by recurrent attacks of acute inflammatory arthritis—a red, tender, hot, swollen joint. The metatarsal-phalangeal joint at the base of the big toe is the most commonly affected . However, it may also present as tophi, kidney stones, or urate...

, and any disease caused by a toxin present in the bloodstream. All of these conditions result from the presence of an increased amount of a particular substance. In ideal circumstances, homeostatic control mechanisms should prevent this imbalance from occurring, but, in some people, the mechanisms do not work efficiently enough or the quantity of the substance exceeds the levels at which it can be managed. In these cases, medical intervention is necessary to restore the balance, or permanent damage to the organs may result.

According to the following quote, every illness has aspects to it that are a result of lost homeostasis:
"Just as we live in a constantly changing world, so do the cells and tissues survive in a constantly changing microenvironment. The 'normal' or 'physiologic' state then is achieved by adaptive responses to the ebb and flow of various stimuli permitting the cells and tissues to adapt and to live in harmony within their microenvironment. Thus, homeostasis is preserved. It is only when the stimuli become more severe, or the response of the organism breaks down, that disease results - a generalization as true for the whole organism as it is for the individual cell." (Pathologic Basis of Disease, third edition, S.L. Robbins MD, R.S. Cotran MD, V.K. Kumar MD. 1984, W.P. Saunders Company)

Varieties


The Dynamic Energy Budget
Dynamic energy budget
The Dynamic Energy Budget theory aims to identify simple quantitative rules for the organization of metabolism of individual organisms that can be understood from basic first principles...

 theory for metabolic organization delineates structure and (one or more) reserves in an organism. Its formulation is based on three forms of homeostasis:
  • Strong homeostasis, whereas structure and reserve do not change in composition. Because the amount of reserve and structure can vary, this allows a particular change in the composition of the whole body (as explained by the Dynamic Energy Budget
    Dynamic energy budget
    The Dynamic Energy Budget theory aims to identify simple quantitative rules for the organization of metabolism of individual organisms that can be understood from basic first principles...

     theory).
  • Weak homeostasis, wherein the ratio of the amounts of reserve and structure becomes constant as long as food availability is constant, even when the organism grows. This means that the whole body composition is constant during growth in constant environments.
  • Structural homeostasis, wherein the sub-individual structures grow in harmony with the whole individual; the relative proportions of the individuals remain constant.

Ecological


The concept of homeostasis is central to the topic of Ecological Stoichiometry
Ecological stoichiometry
Ecological stoichiometry considers how the balance of energy and elements affect and are affected by organisms and their interactions in ecosystems. Ecological stoichiometry has a long history in ecology with early references to the constraints of mass balance made by Liebig, Lotka, and Redfield...

. There it refers to the relationship between the nutrient content of a resource and the nutrient content of its resources. Stoichiometric homeostasis helps explain nutrient recycling and population dynamics.

Historically, ecological succession
Ecological succession
Ecological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...

 was seen as having a stable end-stage called the climax
Climax community
In ecology, a climax community, or climatic climax community, is a biological community of plants and animals which, through the process of ecological succession — the development of vegetation in an area over time — has reached a steady state. This equilibrium occurs because the climax community...

 (see Frederic Clements
Frederic Clements
Frederic Edward Clements was an American plant ecologist and pioneer in the study of vegetation succession.-Biography:...

), sometimes referred to as the 'potential biodiversity' of a site, shaped primarily by the local climate. This idea has been largely abandoned by modern ecologists in favor of nonequilibrium ideas of how ecosystems function, as most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable.

Only on small, isolated habitats known as ecological island
Ecological island
An ecological island is not necessarily an island surrounded by water, but is an area of land, isolated by natural or artificial means from the surrounding land, where a natural micro-habitat exists amidst a larger differing ecosystem....

s can the phenomenon be observed. One such case study is the island of Krakatoa
Krakatoa
Krakatoa is a volcanic island made of a'a lava in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is used for the island group, the main island , and the volcano as a whole. The island exploded in 1883, killing approximately 40,000 people, although some estimates...

 after its major eruption in 1883: the established stable homeostasis of the previous forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...

 climax ecosystem was destroyed, and all life was eliminated from the island. In the years after the eruption, Krakatoa went through a sequence of ecological changes in which successive groups of new plant or animal species followed one another, leading to increasing biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...

 and eventually culminating in a re-established climax community. This ecological succession
Ecological succession
Ecological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...

on Krakatoa occurred in a number of stages; a sere
Sere
Sere may refer to:*Sere , an ecological stage or event, one such event in a series*Sere, Mali*Sere people, an ethnic group in Southern Sudan*SERE, a military training program**United Kingdom - survive, evade, resist, extract...

is defined as "a stage in a sequence of events by which succession occurs". The complete chain of seres leading to a climax is called a prisere. In the case of Krakatoa, the island reached its climax community, with eight hundred different recorded species, in 1983, one hundred years after the eruption that cleared all life off the island. Evidence confirms that this number has been homeostatic for some time, with the introduction of new species rapidly leading to elimination of old ones. The evidence of Krakatoa, and other disturbed island ecosystems, has confirmed many principles of Island Biogeography
Island biogeography
Island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness of natural communities. The theory was developed to explain species richness of actual islands...

, mimicking general principles of ecological succession albeit in a virtually closed system comprised almost exclusively of endemic species.

Biosphere


In the Gaia hypothesis
Gaia hypothesis
The Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life on the planet.The scientific investigation of the...

, James Lovelock
James Lovelock
James Lovelock, CH, CBE, FRS is an independent scientist, environmentalist and futurologist who lives in Devon, England. He is best known for proposing the Gaia hypothesis, which postulates that the biosphere is a self-regulating entity with the capacity to keep our planet healthy by controlling...

 stated that the entire mass of living matter on Earth (or any planet with life) functions as a vast homeostatic superorganism
Superorganism
A superorganism is an organism consisting of many organisms. This is usually meant to be a social unit of eusocial animals, where division of labour is highly specialised and where individuals are not able to survive by themselves for extended periods of time. Ants are the best-known example of...

 that actively modifies its planetary environment to produce the environmental conditions necessary for its own survival. In this view, the entire planet maintains homeostasis. Whether this sort of system is present on Earth is still open to debate. However, some relatively simple homeostatic mechanisms are generally accepted. For example, it is sometimes claimed that when atmospheric carbon dioxide levels rise, certain plants are able to grow better and thus act to remove more carbon dioxide from the atmosphere. However, warming has exacerbated droughts, making water the actual limiting factor
Limiting factor
A limiting factor or limiting resource is a factor that controls a process, such as organism growth or species population, size, or distribution. The availability of food, predation pressure, or availability of shelter are examples of factors that could be limiting for an organism...

 on land. When sunlight is plentiful and atmospheric temperature climbs, it has been claimed that the phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 of the ocean surface waters may thrive and produce more dimethyl sulfide
Dimethyl sulfide
Dimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...

, DMS. The DMS molecules act as cloud condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

, which produce more clouds, and thus increase the atmospheric albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, and this feeds back to lower the temperature of the atmosphere. However, rising sea temperature has stratified the oceans, separating warm, sunlit waters from cool, nutrient-rich waters. Thus, nutrients have become the limiting factor, and plankton levels have actually fallen over the past 50 years, not risen. As scientists discover more about Earth, vast numbers of positive and negative feedback loops are being discovered, that, together, maintain a metastable condition, sometimes within very broad range of environmental conditions.
Environmental pressure, such as competition or change in temperature, can lead to adaptation/extinction of species over time.

Reactive


Example of use: "Reactive homeostasis is an immediate homeostasic response to a challenge such as predation."

However, any homeostasis is impossible without reaction - because homeostasis is and must be a "feedback" phenomenon.

The phrase "reactive homeostasis" is simply short for "reactive compensation reestablishing homeostasis", that is to say, "reestablishing a point of homeostasis." - it should not be confused with a separate kind of homeostasis or a distinct phenomenon from homeostasis; it is simply the compensation (or compensatory) phase of homeostasis.

Risk


An actuary
Actuary
An actuary is a business professional who deals with the financial impact of risk and uncertainty. Actuaries provide expert assessments of financial security systems, with a focus on their complexity, their mathematics, and their mechanisms ....

 may refer to risk homeostasis, where (for example) people that have anti-lock brakes have no better safety record than those without anti-lock brakes, because the former unconsciously compensate for the safer vehicle via less-safe driving habits. Previous to the innovation of anti-lock brakes, certain maneuvers involved minor skids, evoking fear and avoidance: now the anti-lock system moves the boundary for such feedback, and behavior patterns expand into the no-longer punitive area. It has also been suggested that ecological crises are an instance of risk homeostasis in which a particular behavior continues until proven dangerous or dramatic consequences actually occur.

Stress


Sociologists and psychologists may refer to stress homeostasis, the tendency of a population or an individual to stay at a certain level of stress, often generating artificial stresses if the "natural" level of stress is not enough.

Jean-François Lyotard
Jean-François Lyotard
Jean-François Lyotard was a French philosopher and literary theorist. He is well known for his articulation of postmodernism after the late 1970s and the analysis of the impact of postmodernity on the human condition...

, a postmodern theorist, has applied this term to societal 'power centers' that he describes as being 'governed by a principle of homeostasis,' for example, the scientific hierarchy, which will sometimes ignore a radical new discovery for years because it destabilises previously-accepted norms. (See The Postmodern Condition: A Report on Knowledge by Jean-François Lyotard)

Psychological


Author George Leonard
George Leonard
George Leonard was an American lawyer, jurist, and politician from Norton, Massachusetts. Besides service on state court benches and in both houses of the state legislature, he represented Massachusetts in the U.S. House of Representatives.-External links:*...

discusses in his book Mastery how homeostasis affects our behavior and who we are. He states that homeostasis will prevent our body from making drastic changes and maintain stability in our lives even if it is detrimental to us. Examples include when an obese person starts exercising, homeostasis in the body resists the activity to maintain stability. Another example Leonard uses is a unstable family where the father has been a raging alcoholic and suddenly stops and the son starts up a drug habit to maintain stability in the family. Homeostasis is the main factor that stops people changing their habits because our bodies view change as dangerous unless it is very slow. Leonard discusses this dilemma as the media today only encourages fast change and quick results. The opening of his book aptly describes his despair with the current state of the world and how it is at war with homeostasis. "The trouble is that we have few, if any, maps to guide us on the journey or even to show us how to find the path. The modern world, in fact, can be viewed as a prodigious conspiracy against mastery. We're continually bombarded with the promises of immediate gratification, instant success, and fast, temporary relief, all of which lead in exactly the wrong direction."

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